R or S mark–reported as “R” for Repair or “S” for Service. The usage of markings could vary depending on whom did the work. These marks could also be used on guns reworked in the factory before shipping.
Supica, Jim; Nahas, Richard. Standard Catalog of Smith & Wesson, 5th Edition (p. 90). Gun Digest Media. Kindle Edition.
I have never seen the B for "blue" on a postwar revolver. By that time, an N was used for nickel, but blue was standard fare, so the B was not stamped on the barrel flat. The exception was on early units, when a capital B was used to designate the non-standard bright blue.That would designate the original finish for the gun was blued.